Stereo image processing apparatuses which measure a distance to a target (hereafter simply referred to as “stereo image processing apparatus”) are known as conventional technologies. A stereo image processing apparatus extracts, from a reference image, a point at which an object identical to an object captured at a target point in a target image (hereafter referred to as “corresponding point”, and calculates a distance to an object based on a camera parameter and a disparity which represents a shift amount of a corresponding point relative to the target point.
Applications of stereo image processing apparatuses in consideration include a safety apparatus that measures a distance to a forward vehicle or a pedestrian captured by a vehicle-mounted camera. Since the safety apparatuses are incorporated into small-sized cars in recent years, miniaturization of stereo cameras has been required. As the size of stereo camera is reduced, the distance between a camera that captures a target image and a camera that captures a reference image is reduced. The reduction in the distance results in a reduced disparity, which requires a highly precise disparity calculation.
The conventional technique disclosed in PTL 1 is a technology for highly precise disparity calculation. According to the conventional technique, a disparity is calculated using one-dimensional POC (Phase Only Correlation). More specifically, a one-dimensional pixel data sequence is clipped using a Hanning window from each of the target image and the reference image, and is synthesized after performing one-dimensional Fourier transform. The amplitude component of the synthesized data sequence is normalized, and a phase-only correlation coefficient is calculated by one-dimensional inverse Fourier transform. Subsequently, a disparity is calculated based on the peak of correlation of phase-only correlation coefficient.